The present invention relates to video signal scrambling systems and, more particularly, to a coring system for providing a substantially stable color subcarrier signal while retaining full luminance resolution, in line time shifting or position modulating video scrambling apparatus.
The U.S. Pat. Nos. Re. 35,078 (""078), 5,438,620 (""620) and 5,504,815 (""815) of previous mention, incorporated herein by reference, disclose typical line positional shifting video scrambler processes and apparatus. Such line positional or time shifting video scramblers include a video comb filter or equivalent luminance/chrominance separator to separate the composite input signal into luminance and chrominance components (see FIG. 1A) or luminance and demodulated chrominance R-Y and B-Y components (see FIG. 1B). The separated signals then are shifted in time or position via suitable memories, such as first-in-first-out (FIFO) memories, or variable delay lines. In FIG. 1A, the chrominance signal first is time shifted (that is, scrambled) and then is color subcarrier stabilized in both phase and frequency (for example, via a heterodyning process), before being added to the scrambled luminance signal, to provide a composite color stabilized time or position modulated scrambled video output signal. In FIG. 1B, where the R-Y and B-Y demodulated chrominance components are provided, the time shifted or position modulated R-Y and B-Y components are re-encoded with a stable color subcarrier frequency and phase before being added to the time shifted or position modulated luminance signal to provide a composite color stabilized time shifted or position modulated scrambled video output signal.
However, the video comb filter or luminance/chrominance separator of the above scrambling systems does not provide perfect separation of the chrominance (chroma) and luminance (luma) components. Therefore residual chroma remains in the luma channel in the scrambling systems described above. Normally, imperfect luminance-chrominance separation is not a problem in equipment such as television sets. That is, in such equipment, the residual chrominance in the luminance channel still is stable chrominance and thus does not contribute to chrominance instabilities in, for example, the television set. However, once the luma channel is time shifted by the scrambling process, the residual chroma becomes unstable in phase and frequency. When the time shifted or (low frequency) position modulated luminance channel signal with the unstable residual chroma is added to the stabilized time shifted chroma channel signal, a composite video signal is produced with small, but visible, unstable chroma phase and amplitude errors when scrambled and later descrambled. These unstable chroma phase and amplitude errors cause low frequency color streaking or hue and saturation noise throughout the television field.
FIG. 1A illustrates a basic video path for a scrambling system 10 which employs a signal wobbling technique, such as described in the ""620 and ""815 patents of previous mention. A program video signal, such as a composite video signal, is supplied via an input lead 12 to a comb filter 14. The elements 76, 78 and 80 shown in FIG. 4 of the ""620 and ""815 patents exemplify elements which can be used to form the comb filter 14 in FIG. 1A. The comb filter 14 provides outputs of a luma signal with residual chroma, and a chroma signal. The luma signal along with its residual chroma are supplied to a time shift element 16 to effect the scrambling process, whereby the element 16, provides a shifted luma signal with shifted unstable phase residual chroma. The chroma signal is supplied to a second time shift element 18 which provides the previously mentioned scrambling of the chroma signal. It is to be understood that the time shift elements 16, 18 could be position modulation elements as well. Both of the time shift (or position modulation) elements shift the video signal by an equal amount as part of the particular scrambling process used. The output of the time shift element 18, comprising the time shifted chroma signal, is supplied to a chroma subcarrier stabilizer 20. The chroma subcarrier stabilizer 20 is comparable to the hetrodyne element 100 in FIG. 4 of the ""620 and ""815 patents, and provides a shifted chroma signal with stable phase. The outputs of the chroma subcarrier stabilizer 20 and of the time shift element 16 are supplied to the inputs of an adder circuit 22, which produces a scrambled video signal having chroma subcarrier instabilities on an output lead 24.
FIG. 1B illustrates a basic video path for a scrambling system 30 which employs a signal wobbling technique such as described in the ""078 patent of previous mention. A program video signal, such as a composite video signal, is supplied via an input lead 32 to a comb filter/chroma demodulator circuit 34. The element 16 shown in FIG. 4A of the ""078 patent exemplifies an element which can be used as the comb filter/chroma demodulator circuit 34 in FIG. 1B. The comb filter/chroma demodulator circuit 34 provides outputs of a luma signal with some residual chroma, a R-Y component and a B-Y component. The luma signal along with its residual chroma are supplied to a time shift or position modulation element 36, which provides a scrambled luma signal formed of a shifted luma signal with shifted unstable phase residual chroma. The R-Y and B-Y signals are supplied to a two channel time shift element 38 which provides scrambled R-Y and B-Y component signals. The time shift elements 36, 38 shift the respective video signal by an equal amount as part of the scrambling process. The outputs of the time shift element 38, comprising the time shifted or position modulation R-Y and B-Y signals, are supplied to a chroma encoder 40. The chroma encoder 40 is comparable to the encoder element 25 in FIG. 4A of the ""078 patent of previous mention, and provides a scrambled chroma signal with stable phase. The outputs of the chroma encoder 40 and of the time shift element 36 are supplied to the inputs of an adder circuit 42, which produces a scrambled video signal having chroma subcarrier instabilities on an output lead 44.
One solution for removing the unstable residual chroma from the positionally shifted luma channel includes the application of a notch filter in the luminance channel, wherein the filter has a frequency band around the color subcarrier frequency and its color sidebands. However, this solution severely degrades the luminance resolution and thus degrades the frequency response of the video scrambler""s and thus the descrambler""s output. The degraded resolution eliminates much of the advantage that video comb filters provide, since comb filters normally aid in gaining full luminance resolution. Accordingly, there is a need for a solution that eliminates the unstable chroma phase and amplitude errors without degrading the luminance frequency response and the resultant resolution of the signal scrambling system.
An object of the present invention is to retain full luminance resolution of video scrambling systems while providing a substantially stable color subcarrier. The stable subcarrier phase and amplitude appear in the resulting composite video output signal, corresponding to the time shifted or position modulated scrambled video signal. The increased stability substantially reduces or eliminates chroma noise due to imperfect luminance/chrominance signal separation. That is, it is the intent of the present invention to substantially remove the unstable residual chroma color subcarrier caused by the small amount of chroma signal which leaks into the luma channel, so when the signal in the positionally modulated or time shifted (wobbled) luminance channel is summed with the signal in the positionally modulated or time shifted stable chroma color subcarrier channel, color instabilities are substantially reduced to a minimum.
To this end, the method and apparatus of the present invention employs a chroma coring system to remove the unstable residual chroma color subcarrier, that is, the unstable residual chroma, in the luma channel. An alternative embodiment of the invention provides further improvement by including an adaptive chroma coring system. The adaptive chroma coring is provided by variably adjusting the amount of coring applied in accordance with the chroma amplitude sensed in the chrominance signal in the scrambled stabilized chroma color subcarrier channel, or in the signal derived from a video comb filter in the chrominance channel. For example, if the program video input signal has large areas of highly saturated color components, the chroma coring is electronically turned up. At the other extreme, if the program video signal is essentially in black and white (no color content), then the coring circuit essentially is electronically turned off. Thus, the adaptive coring technique maximizes the frequency and pulse response for black and white signals.
It is to be understood that the invention also can be used to enhance the luminance-chrominance separation of video comb filters frequently used in other video circuits and systems such as television sets and video recorders.
These and other aspects, features and advantages of the invention will become apparent upon review of the succeeding description taken in connection with the accompanying drawings.